4

Changes in the Climate System on Decade-to-Century Timescales

SUMMARY

Research on changes in the climate system on decade-to-century timescales has achieved notable successes in the past decade. The effective use of the paleoclimate record has revealed attributes of natural climate variability and has provided a context for the study of present and future global change. Findings about rapid climate change have been particularly enlightening, such as the recent recognition of decadal patterns in the atmosphere. This discovery is owed mostly to analyses of long-term, upper-air data, demonstrating the essential value of maintaining such long-term consistent records.

Recent advances in understanding climate prediction on timescales of decades to centuries include the following, among others: documentation and recognition of the scope of natural variability; documentation by calibrated satellite observations that clouds have a net global radiative cooling effect on the Earth-atmosphere system by about 15 to 20 W/m2; achievements in understanding water vapor behavior and in feedback analysis, proposed and to some degree realized, on theoretical, observational, modeling, and methodological grounds; and understanding the role of volcanic eruptions as a climate-forcing factor, as seen in measurement and assessment of the impacts of recent eruptions.

This area of research has underscored the complexities and uncertainties of detecting and projecting climate change. It has become even clearer that determining the roles of anthropogenic forcing is inseparable from understanding the natural system. Anthropogenic global change cannot be assessed without adequate understanding and documentation of natural climate variability on timescales of years to centuries —in other words, without adequate baseline understanding. This understanding encompasses solar and volcanic variability; feedbacks

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Global Environmental Change: Research Pathways for the Next Decade . Washington, DC: The National Academies Press,
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4
Changes in the Climate System on Decade-to-Century Timescales
SUMMARY
Research on changes in the climate system on decade-to-century timescales has achieved notable successes in the past decade. The effective use of the paleoclimate record has revealed attributes of natural climate variability and has provided a context for the study of present and future global change. Findings about rapid climate change have been particularly enlightening, such as the recent recognition of decadal patterns in the atmosphere. This discovery is owed mostly to analyses of long-term, upper-air data, demonstrating the essential value of maintaining such long-term consistent records.
Recent advances in understanding climate prediction on timescales of decades to centuries include the following, among others: documentation and recognition of the scope of natural variability; documentation by calibrated satellite observations that clouds have a net global radiative cooling effect on the Earth-atmosphere system by about 15 to 20 W/m2; achievements in understanding water vapor behavior and in feedback analysis, proposed and to some degree realized, on theoretical, observational, modeling, and methodological grounds; and understanding the role of volcanic eruptions as a climate-forcing factor, as seen in measurement and assessment of the impacts of recent eruptions.
This area of research has underscored the complexities and uncertainties of detecting and projecting climate change. It has become even clearer that determining the roles of anthropogenic forcing is inseparable from understanding the natural system. Anthropogenic global change cannot be assessed without adequate understanding and documentation of natural climate variability on timescales of years to centuries —in other words, without adequate baseline understanding. This understanding encompasses solar and volcanic variability; feedbacks

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resulting from the interactions of water vapor, clouds, and radiation; and the massive heat fluxes associated with the motions of the air and oceans and the exchanges between them, among other phenomena, beyond quantified understanding of anthropogenic forcing itself. To evaluate anthropogenic forcing specifically, greater knowledge is also needed of tropospheric aerosols and the carbon cycle.
The primary characteristics of the climate system must be documented through consistent long-term observations. Finally, the subtlety of slow change over long timescales, in contrast to diurnal, seasonal, and interannual variations, can disguise its potential long-term severity and thus limit society's willingness to address potential problems in advance. The problem is much exacerbated, of course, by the uncertainty in our ability to forecast such change. All these considerations further underscore the importance of achieving better understanding of climate change patterns on decade to century timescales, including their rate and range of variability, likelihood and distribution of occurrence, and the sensitivity of climate to changes in forcing (natural and anthropogenic). With such improved understanding, we ultimately hope to forecast and detect change (distinguishing natural from anthropogenic), providing a foundation on which future policy decisions and infrastructure management can be rationally based.
A number of Research Imperatives must be met to understand climate change on decadal to centennial timescales:
Natural climate patterns. Improve knowledge of decadal- to century-scale natural climate patterns, their distributions in time and space, optimal characterization, mechanistic controls, feedbacks, and sensitivities, including their interactions with, and responses to, anthropogenic climate change.
Paleorecord. Extend the climate record back through data archeology and paleoclimate records for time series long enough to provide researchers with a better database to analyze decadal- to century-scale patterns. Specifically, achieve a better understanding of the nature and range of natural variability over these timescales.
Long-term observational system. Ensure the existence of a long-term observing system for a more definitive observational foundation to evaluate decadal- to century-scale variability and change. Ensure that the system includes observations of key state variables as well as external forcings.
Climate system components. Address those issues whose resolution will most efficiently and significantly advance our understanding of decadal- to century-scale climate variability for specific components of the climate system.
Anthropogenic perturbations. Improve understanding of the long-term responses of the climate system to the anthropogenic addition of radiatively active constituents to the atmosphere and devise methods of detecting an-

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thropogenic phenomena against the background of natural decadal-to century-scale climate variability.
INTRODUCTION
Climate research on decade to century (“dec-cen”) timescales is relatively new. Only recently have we obtained sufficient high-resolution paleoclimate records, and acquired faster computers and improved models allowing long-term simulations, to examine past change on these timescaies. This research has led to genuinely novel insights, most notably that the past assumption of a relatively stable climate state on dec-cen timescales since the last deglaciation is no longer a viable tenet. The paleorecords reveal considerable variability occurring over all timescales, while modeling and theoretical studies indicate modes of internal and coupled variability driving variations over dec-cen timescaies as well.
Thus, dec-cen climate research is only at the beginning of its learning curve, with dramatic findings appearing at an impressive rate. In this area even the most fundamental scientific issues are evolving rapidly. Adaptability to new directions and opportunities is therefore imperative to advance understanding of climate variability and change on these timescales.
The paradigm developed to successfully study climate change on seasonal to interannual timescales cannot be applied to the study of dec-cen climate problems. That is, we have realized considerable success studying short timescale climate problems by generating hypotheses and models that are quickly diagnosed and improved based on analysis of the amply long historical records or quickly realized future records. For dec-cen problems the paleoclimate records are still too sparse and the historical records too short. Future records will require multiple decades before even a nominal comparison to model predictions is possible. Compounding the problem, the change in atmospheric composition as a consequence of anthropogenic emissions represents a forcing whose future trends can only be estimated with considerable uncertainty. As a result, progress requires considerable dependence on improved and faster models, an expanded paleoclimate database, and imposed (rather than calculated) anthropogenic emission scenarios. Heavy reliance on these methods and assumed forcing curves, without the benefit of real-time observations for constant model validation and improvement, implies a considerable effort for model validation through alternative means, improved understanding of the limits and implications of proxy indicators constituting the paleoclimate records, and detailed monitoring of emissions to help track actual rates. As for future observations, we can only now begin collecting the data to aid future generations of scientists in understanding dec-cen climate variability and change.
Climate variability and change on decade to century timescales involves all of the elements of the U.S. Global Change Research Program: natural and anthropogenic variability and change; past, present, and future observational networks

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and databases; modeling requirements; and physical, chemical, biological, and social sciences, with considerable attention to the human dimensions of climate change. The last focus is particularly important on dec-cen timescales because the magnitude of change is often, though not always, proportional to the timescale over which it varies. Consequently, climate change over these long timescales could produce much greater social, economic, and political impacts than shorter timescale variations, which are often addressed through disaster relief. On deccen timescales the impacts could be considerable, and adaptation and mitigation (of both the forcing and response) depend on policy decisions and investments in infrastructure. For example, the devastating floods that struck the Midwestern United States in 1993 and again in 1997 produced considerable hardship, loss, and destruction, requiring substantial recovery aid. However, if we knew that such floods occurred in, say, clusters of six or seven over a 20-year period, such information might dramatically reduce the negative impacts, through mitigation actions in policy and infrastructure. Perhaps we could even benefit in some ways from these events. Similar action would be possible, given advanced knowledge of the frequency or magnitude of extreme heat days for any particular region or, for that matter, knowledge of any other changes that might greatly affect agriculture, energy production and use, water resources and water quality, air quality, health, fisheries, forestry, insurance, recreation, and transportation. All of these areas are fundamental to society's well-being and would certainly be affected by any prolonged or abrupt shift in our climate system.
Unfortunately, the subtlety of slow change over long timescales, relative to diurnal, seasonal, and interannual variations, can disguise the potential severity of longer-term change and thus limit society 's willingness to address the issues in advance. This difficulty underscores the importance of better understanding of decadal- to century-scale climate change, its rate and range of variability, its likelihood and distribution of occurrence, and its sensitivity to changes in forcing (natural and anthropogenic). With such understanding we may ultimately forecast and detect change (distinguishing natural from anthropogenic), providing a foundation for more rationally based policy decisions and infrastructure management.
CASE STUDIES
The four case studies presented below all relate to issues of dec-cen climate variability. The first case reviews findings from Greenland ice cores about the natural variability of the climate system. The second illuminates human responses to climate variability in Mesopotamia, as deduced from the paleorecord. A case of modern response to climate change is then described, concerning flood control on the American River near Sacramento. The fourth and final case study discusses emerging signals of the human-influenced climate system.

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Natural Variability
The prediction and modeling of future climate change and its effects on the environment and people are two of the most challenging tasks facing science today. To understand possible future changes in climate, knowledge of past climate change is essential. As explained in Chapter 6, ice cores were recovered in 1992 after a five-year drilling effort in the Summit region of Greenland by the U.S. Greenland Ice Sheet Project Two (GISP2) and from the European project GRIP (Greenland Ice Core Project, sited 30 km to the east of the GISP2 site); and they have produced an unparalleled record of climatic change for the past 110,000 years.a
The cores revealed changes in the Earth's climate system over the past 150,000 years or so, with annual resolutions over the past several thousand years. One of the most remarkable findings from these cores was that the climate during the past several thousand years—the period we would consider modern climate—has undergone considerable natural variability, including large swings or cycles of climate and, even more remarkably, abrupt changes occurring in decades or less. In addition to these findings, the long record of climate change also suggests that, relative to earlier times in the Earth's climate history, these past several thousand years have shown relatively little variability in climate change. The implication is that the impressive, and often abrupt, swings in climate recorded over the past several thousand years may, if anything, understate the potential for natural climate variability.
The Summit region has proven to be an ideal site from which to recover deep ice cores. The approximate −31°C mean annual air temperature there and the minimal occurrence of melt layers throughout the record assure the in situ preservation of a broad range of gaseous, soluble, and insoluble measurements of the paleo-environment. Similarity of the GISP2 and GRIP records is compelling evidence that the stratigraphy of the ice is reliable and unaffected by extensive folding, intrusion, or hiatuses from the surface to 2,790 m (~110,000 years ago). This agreement between the two cores strongly supports the climatic origin of even minor features of the records and suggests that investigations of subtle environmental signals (e.g., rapid climate change events with one- to two-year onset and termination) can be rigorously pursued.
a
GISP2 successfully completed drilling through the base of the Greenland ice sheet and another 1.55m into bedrock in central Greenland on July 1, 1993, recovering the deepest ice core record in the northern hemisphere (3053.44m). GISP2, a component of Arctic System Science, is comprised of investigators from 22 institutions. Twenty programs with 46 types of measurements on the ice core comprise the deep drilling effort. Nine other programs provide direct information necessary for interpretation of the GISP2 ice core record.

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A Distant Past: The Younger Dryas and Other Rapid Climate Change Events Over the Past 110,000 Years
The Younger Dryas was the most important rapid climate change event that occurred during the last deglaciation of the North Atlantic region. Previous ice core studies had focused on the abrupt termination of this event because this transition marks the end of the last major climate reorganization during the deglaciation. Most recently, the Younger Dryas has been redated, using precision, subannually resolved, multivariate measurements from the GISP2 core, as an event of 1,300 +/−70 years' duration that terminated abruptly at 11,640 years before the present (BP), as evidenced by a rise in temperature of about 7°C and a twofold increase in the snow accumulation rate. The transition into the Preboreal, the Preboral/Younger Dryas transition, and the Younger Dryas/Holocene transition were all remarkably fast, each occurring over a decade or less (see Chapter 6).
The isotopic temperature records show 23 interstadial (or Dansgaard/Oeschger) events, first recognized in the GRIP record and verified in the GISP2 record, between 110,000 and 15,000 years BP. These millennial-scale events represent quite large climate deviations—probably of many degrees in temperature, twofold changes in snow accumulation, order-of-magnitude changes in wind-blown dust and sea salt loading, and roughly 100 ppb (volume) swings in atmospheric methane concentration.
In view of all these measures, the events must have been regional to global in scale. They are seen in local climatic indicators, such as snow accumulation rate and isotopic composition of snow linked to temperature; in regional climatic indicators, such as wind-blown sea salt and continental dust; and in regional to global indicators, such as atmospheric concentrations of methane, nitrate, and ammonium. Some of the events are also readily identified in the ocean-sediment record in regions critical to global ocean circulation.
Since these cores were obtained, additional investigations, involving large numbers of proxy indicators of past climate change, from all of the different climate zones on Earth, have reinforced these initial findings and more clearly driven home the vulnerability of the Earth 's climate system to natural variability. Consequently, these findings have changed our way of viewing the climate system and fundamentally undercut the notion that we live in a relatively stable climate system.
The Last 500+ Years: The Little Ice Age, Medieval Warm Period, and Fossil Fuel Era
The Little Ice Age and Medieval Warm Period environments are the most recent analogs for conditions cooler and warmer, respectively, than the present century. Each period can be characterized by interpreting the multiparameter

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GISP2 series (e.g., CO2, stable isotopes, major ions, accumulation rate, particles). GISP2 temperature modeled from oxygen isotopes reveals a relatively subdued temperature effect at this Greenland site for the Little Ice Age. More recently, year-to-year correlations between the GISP2 isotopic record and sea surface and land temperatures over the North Atlantic, covering the period 1840 to 1970, reveal changes in atmospheric circulation patterns, such as the seesaw pattern of the North Atlantic Oscillation, demonstrating the sensitivity of the isotopic record.
Levels of continental dusts and marine sea salts increased during the Little Ice Age in response to increased meridional circulation. The Little Ice Age is one of several glaciochemically identifiable climate events in the Holocene record that correlate with other paleoclimate records. The period is characterized by the most rapid onset of any Holocene cold period.
Measurements of CO2 in air bubbles of the GISP2 core indicate that between 1530 and 1810 atmospheric CO2 levels remained relatively constant at +/−280 ppm(v). Thereafter, concentrations rose rather abruptly and smoothly connected to the atmospheric observations at Mauna Loa. Previously identified increases in sulfate and nitrate seen in south Greenland ice cores and attributed to anthropogenic activity were identified in the GISP2 core and contrasted to the preanthropogenic atmosphere. An observed increase in chloride at GISP2, as in the 1940s, is believed to be a byproduct of increased anthropogenic HNO3 and H2SO4, since these compounds are believed to aid in the volatilization of HCl from sea salt aerosol.
Human Responses to Climate Change as Deduced from the Paleorecord
Although the issue of human response to climate change is controversial, several recent studies find close correlations in timing between climate change and changes in civilization. These studies have focused on changes in temperature in relation to high-latitude societies and changes in moisture availability for mid- to low-latitude societies. In regions on the ice margins, such events as the disappearance of the Norse colonies in Greenland during the mid- to late fourteenth century appear to be chronologically correlated at some sites with the occurrence of a few extremely cold winters and at others with the general amelioration of climate produced at the onset of the Little Ice Age.1
By utilizing climate-linked paleoclimate records, it was found that periods of decreased atmospheric circulation intensity in the North Atlantic, developed from the GISP2 ice core, could be correlated with discontinuous Dead Sea level records of drying,2 which are a reasonable indicator for west Asian aridity.3; The more detailed record reveals a close correlation between major periods of drying and major social disruptions in west Asian civilization. 4 Other research5 has found that the driest period represented by a late Holocene lake sediment record from Mexico correlates closely with the collapse of the classic Mayan civilization around 750 to 900 AD.

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A Modern Climate Change Dilemma: Flood Control on the American River
The significance of decadal- to centennial-scale climate variability is highlighted by a recent example of water resources planning.6 Flood control projects are designed to protect facilities from a design flood or flow. The level of protection (i.e., the risk of project failure) provided against the design flood is assessed through statistical analysis of the historical flood record. The economics of a new flood control project are determined by comparing the expected monetary benefits of reducing flood risk and the associated project cost. Flood insurance programs rely on a similar analysis. The variability of flood risk at decadal to centennial timescales and its implications for flood control are discussed here in the context of the American River near Sacramento, California.
Flood protection for Sacramento is provided by the Folsom Dam together with a system of levees. The dam was designed in the late 1940s, based in part on a flood record extending back to 1905. Since the dam's design, there have been six floods (not including the 1997 flood) on the American River larger than all previously recorded floods (see Figure 4.1). The estimated frequency of exceedance of extreme floods has correspondingly increased. It now appears that a large part of Sacramento may not even have 100-year flood protection. Should new flood mitigation projects be based on an assessment of flood risk from the
FIGURE 4.1 The time-varying probabilty of exceeding the 10th, 25th, 50th, 75th, and 90th quantiles of the full American River annual maximum flood record (shown as o), estimated by smoothing (with a 56-year span) a binary indicator (1 = exceedance, else 0) applied to the quantile. Note the trend reversal since about 1940, with an increase in the probability of exceedance of the rarer floods and a decrease for the more common floods. SOURCE: National Research Council (1995a).

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FIGURE 4.2 Date of annual maximum flood for the American River near Fair Oaks. Centennial and decadal trends are shown by the solid (56-year smooth) and the dotted lines (14-year smooth). SOURCE: National Research Council (1999).
entire flood record or from the past 50 years? A project designed to provide a 200-year level of protection based on the full flood record would provide less than 100-year protection based on the record since 1950. Project costs and potential flood damages could vary by over an order of magnitude depending on the protection level adopted. This decision-making dilemma was noted by the National Research Council Committee on Flood Control Alternatives in the American River Basin. 7
Since about 1940, the annual maximum flow on the American River has also occurred earlier in the year (see Figure 4.2), with a decadal fluctuation superposed on this trend. This pattern has implications for the types of models (e.g., rain on snow dynamics instead of rainfall runoff) needed for flood forecasting and for real-time flood control. A number of factors, including improvements in streamflow measurement technology and urbanization of the watershed, may be responsible for these changes in the flood regime. However, structured decadal to centennial climate variations are a likely cause.
Others8 argue that earlier snowmelt in California may be caused by a trend toward warmer winters there and a concurrent long-term fluctuation in winter atmospheric circulation over the North Pacific Ocean and North America. The fluctuation began to affect California in the 1940s, when the region of strongest low-frequency variation in winter circulations shifted to a part of the central

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North Pacific Ocean that is strongly linked to California temperatures through the Pacific-North American (PNA) teleconnection pattern. 9 Since the late 1940s, winter wind fields have been displaced progressively southward over the central North Pacific and northward over the West Coast of North America. These shifts in atmospheric circulation are associated with concurrent shifts in both West Coast air temperatures and North Pacific sea surface temperatures and with earlier snowmelt and increased spring moisture fluxes in the American River basin.
Gridded (5° * 5°) monthly records of northern hemisphere sea level pressure (SLP) 10 and surface temperature11 for the period 1899 to 1996 have been used to reconstruct space and time patterns of quasi-oscillatory large-scale climate patterns at quasi-biennial ENSO (El Niño-Southern Oscillation), decadal, interdecadal, and secular frequency bands.12 For the analysis a 40-year moving window Multi-Taper Method/Singular Value Decomposition (MTM-SVD) was used. Simultaneous analyses of these datasets help identify dynamically consistent space- and time-coherent patterns of low-frequency climate evolution. Projections of the hemispheric low-frequency patterns of SLP and temperature at the grid point closest to the American River streamflow gauge are shown in Figure 4.3. The low-frequency SLP and temperature projections are obtained from the MTM-SVD analysis by summing over the reconstructions for the secular (>30
FIGURE 4.3 The secular and low-frequency components of SLP and temperature at the grid point nearest the American River from MTM-SVD. Note the secular trend toward warmer temperatures and lower pressure in the region, post-1940, coincident with the increased flood incidence and shift in flood timing. SOURCE: National Research Council (1999).

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years), interdecadal (18-year period), decadal (10-year period), ENSO (3- to 6-year period), and quasi-biennial (2.2-year period) bands at the closest grid point. Note the secular trend for a shift to a lower SLP and warmer temperature at the American River region since about 1940. A remarkable connection between low-frequency climate and the high-frequency flood process is shown. Understanding and long-lead prediction of these fluctuations and their impact on regional hydrology and floods are key for dynamic flood risk assessment and better flood protection design and management. Flood insurance programs could be made much more efficient if long-term regional flood risk could be better assessed and “opposing” trends exploited.
Anthropogenic “Greenhouse” Warming
In 1896 Arrhenius pointed out that the increased concentration of CO2 in the Earth's atmosphere, introduced by the burning of fossil fuels and compounded by other societal byproducts, could enhance the Earth's natural greenhouse warming, leading to an anthropogenic warming of the climate system and affecting civilization throughout the globe. A significant amount of research has been directed toward this problem, to understand if and how such an impact could be realized (or negated by natural feedbacks) and how to detect and interpret the source of such a warming. One of the most perplexing aspects of this research has been understanding the warming that the Earth has indeed experienced over this last century (see Figure 4.4) to determine whether this warming is natural, anthropogenic, or some combination of the two.
As noted in the Intergovernmental Panel on Climate Change (IPCC) Second Assessment (1996), the focus of recent climate change and variability research has shifted from the analysis of mean global temperature to that of temperature spatial distributions. This shift reflects the expectation that climate change may manifest itself irregularly in space and time. For example, it is clear that the relatively rapid global warming experienced over the past 20 to 25 years is distinguished by enhanced warming in winter (not evident in previous decades), with a strong warming over northern hemisphere land, but some small cooling over the northern hemisphere oceans. 13 This is the so-called COWL pattern: cold oceans and warm land pattern that is readily apparent in the global surface temperature data when comparing the past 20 years to the previous 20 years (see Plate 5).
The COWL pattern is a northern hemisphere winter phenomenon. A similar geographic pattern is simulated by numerous anthropogenic modeling studies and thus considered by some to represent one component of the so-called greenhouse fingerprint14—that is, a characteristic of the changing climate that might be uniquely associated with anthropogenic warming, as opposed to natural warming. Its presence in the actual observations has therefore been accepted as additional evidence of anthropogenic warming.15

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As the concentration of greenhouse gases increases in the atmosphere, the atmosphere clearly must respond in some manner to accommodate the change in radiative forcing. The atmosphere may respond by warming to some degree, it may change its vertical distribution of moisture and cloud cover, or any combination of these may occur. Each of the state variables must be monitored, including their vertical distributions through the troposphere and lower stratosphere, to evaluate the nature of anthropogenic and natural changes. One of the most hotly debated topics in modern climatology is how atmospheric moisture distribution will change in response to the addition of greenhouse gases and therefore whether, or by how much, this moisture response will moderate the temperature response. Thus, it is not enough to measure temperature, simply because temperature has been the initial focus of the greenhouse debate.
Atmospheric observations must be colocated with those stations established to monitor surface conditions. This need directly follows from the earlier point that most, if not all, dec-cen atmospheric variability and change are in response to changes in slower components of the climate system, such as land, ice, and ocean. These components represent the lower boundary of the atmosphere. In many cases, as noted above, atmospheric changes strongly covary with changes at the surface. To evaluate, diagnose, and attribute dec-cen change, such covariation must be captured in a manner that facilitates analysis and evaluation of hypotheses that describe the coupled mechanisms driving and modulating long-term variability.
Process studies and related field efforts must be directed to improving our understanding and parameterization of surface-atmosphere interaction. Obviously, it is through this boundary interaction that slower-scale components communicate their influences to the atmosphere. Thus, appropriate parameterization of these phenomena are essential, since modeling efforts are the primary tool we have for forecasting future change. We also need better parameterization of clouds, including distribution and feedback processes, since their treatment in models may prove crucial in predicting long-term climate responses to changes in radiative forcing, as well as other feedback influences associated with variability and change. These parameterizations are currently a primary limitation in existing models.
The Chemical System
The radiative effects of aerosols, direct and indirect, are poorly constrained. Cloud processes, although they occur on far shorter than decadal timescales, are a major uncertainty in predicting future radiation balances. Parameterizations need to be improved.
Carbon cycle questions require a CO2 measurement strategy that accounts for the hierarchy of scales, both temporal and spatial, inherent in ecosystem processes and their controls. Atmospheric concentration data must allow the identification and quantification of regional sources and sinks and their responses

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to climate fluctuations and human perturbations. This information will permit integration over regional scales of fluxes and feedback processes that can be measured, understood, and modeled on smaller spatial and temporal scales. Isotopic data allow distinguishing between oceanic and biospheric sinks on regional scales and have provided significant insight into the regional carbon balance. Ratios of O 2 to N2 in the global atmosphere provide an independent constraint on the balance between net terrestrial and oceanic sinks. The same scaling and measurement issues are almost identical for N2O and CH4, and their biogeochemical budgets can be tackled together with a measurement program suitable for CO2.
Enormous progress in assessing trace gas budgets could be achieved if a method could be developed or refined to directly measure air-sea gas exchange rates. Promising methods are air measurements with eddy correlation and/or eddy accumulation. Such measurements would eventually lead to a realistic understanding of the processes controlling the rate of gas exchange and therefore to a parameterization that could be applied with confidence worldwide. Existing climatologies of the partial pressure differences between the air and the water for many gases could then be turned into maps of gas exchange, making oceanic data into a much more compelling constraint on the atmospheric budget and closing the open boundary of surface oceanic gas budgets.
Ocean Observations
Various types of ocean observations are needed to study the dec-cen variability associated with the primary known patterns of atmospheric climate variability: periodic (decadal) temperature, salinity, oxygen, and tracer sections; velocity profile surveys and repeat sections (starting with World Ocean Circulation Experiment sections); and higher-frequency time series stations (starting with past and present weather ship stations). These measurements will allow better quantitative description of the ocean's participation in that dec-cen variability, especially in light of the slowly propagating SST and subsurface anomalies that have revealed the ocean's dec-cen variability as more than stationary patterns. We must extend these surveys into southern hemisphere regions as the nature of the dec-cen variability begins to be revealed.
These sections and time series stations provide the baseline against which the long-term response and change of the ocean can be measured and the basic observational set from which serendipitous discoveries about the ocean's role in climate change have been realized. In addition, the time series data have been invaluable in studying the ocean' s response to atmospheric forcing and its feedback to the atmosphere. These findings are of particular importance because surface layer interaction and response dictate the volume of water in direct communication with the atmosphere. Even a small change in this volume can lead to a significant change in SST, given the same magnitude of surface forcing. The

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time series stations are the only series available that allow appropriate development, diagnosis, and improvement of these parameterizations.
Continued satellite data are needed for global coverage of sea surface height, SST, winds, and ocean color, but for these data to be useful, corresponding ground-truth ocean observations also are needed. Particular data of interest concern the heat budget. A concerted effort is required to improve estimates of heat flux divergence and heat storage and their variabilities from subsurface ocean data, eliminating disparities between those estimates and air-sea heat exchange estimates. Various subsurface floats and moorings are particularly helpful to supplement shipboard measurements for this study.
Sea level change is another important observational challenge. The IPCC (1996) estimates that in the year 2000 sea level will be 46 to 72 cm higher than today (36 to 53 cm, when the effects of sulfate aerosols are included). A range is given because each projection presumes a specific scenario for increase in greenhouse gasses. To validate these predictions, better monitoring of global sea level change and its components will be needed. The prospects for sea level monitoring are good. A global network of sea level stations (Global Sea Level Observing System) is being implemented. Land movements will be measured at some of these stations with satellite geodesy and gravimetric techniques. Satellite altimetry is another important tool coming into use to measure global sea level rise.
Cryosphere Observations
Critical cryosphere-related observations for climate patterns on decadal to centennial timescales include long-term monitoring of surface salinity along with SST, since salinity represents the dominant control on the density of seawater in high-latitude regions. Also, measurements of the sea ice fields themselves, including motion fields and ice thickness, are required to determine the freshwater transports and buoyancy fluxes associated with the ice fields. This freshwater transport has been implicated in driving major changes, even mode shifts in the global thermohaline circulation. Finally, consistent monitoring of iceberg calving and an observational system for determining ice basal melt or growth (e.g., through temperature/salinity moorings across the floating ice shelves) must be established to better determine the freshwater budget. Both field and satellite studies are needed to refine the mass budgets of the Greenland and Antarctic ice sheets. Onsite studies that focus on ice flow, melting, and calving should be continued and extended. Water vapor flux divergence observations will help pin down the source of the ice sheets' mass. A laser altimeter on a polar-orbiting satellite is needed to augment existing radar altimetry. These satellite data will provide accurate estimates of ice sheet volume and give early warning of possible ice sheet collapse. As in the case of ice, the distribution of snow fields, including thickness and spatial extent must be monitored. The response of snow distribu-

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tion to climate change has been hypothesized as being important in surface-climate feedbacks as well as in climate change diagnostics.
Finally, the ocean-atmosphere-ice interaction, particularly the ice or snow surface energy balance (including surface albedo and ocean-ice, ice-cloud, and snow-cloud feedbacks), must be addressed through detailed process studies to improve parameterizations of these processes in climate models.
Land and Vegetation Observations
As explained in sections above, it is also essential to monitor changes in land surface characteristics, including surface vegetation. These changes alter not only the distribution of surface reservoirs and the surface-atmosphere exchange of radiatively active gases but also albedo and even surface stress and evapotranspiration efficiency —and the last two both influence the hydrological cycle. This serves as an external forcing to the planet that cannot be predicted and must be introduced into the models as they occur to properly maintain the models' surface forcing conditions.
Long-term monitoring of near-surface aerosol distributions also is needed. These distributions may induce stationary changes in the surface radiation balance, which may lead to large-scale circulation moderation through stable gradient perturbations.
Hydrological Observations
Precipitation is the key hydrological variable. For most studies of dec-cen variability and its effects, global fields of precipitation over timescaies of 10 to 100 years are essential. We have no such global instrumental records currently. The National Aeronautics and Space Administration's Tropical Rainfall Monitoring Mission is an important first step, but global data are needed. To relate precipitation to global boundary conditions, SST, vegetative ground cover and soil moisture, and sea and land ice and snow must be simultaneously measured. Nearly every theory of anthropogenic warming finds an increased rate of the hydrological cycle and possible alteration of atmospheric distributions of moisture and of the frequency, intensity, and distribution of rainfall (including severe rainfall events). Thus, monitoring of the surface distribution of precipitation and evaporation must begin. This monitoring includes that over the oceans, where changes in the precipitation minus evaporation balance alter the surface salinity budget, which in high latitudes has been implicated in altering the thermohaline circulation (and driving internal oscillations on dec-cen timescaies in ocean models).

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GLOBAL ENVIRONMENTAL CHANGE: Research Pathways for the Next Decade
CONCLUSIONS
Above we observed that the dec-cen paradigm must differ from that used to study shorter-timescale variability. Moreover, even the nature of the observations collected for dec-cen studies must differ. While, for example, atmospheric state variables must be monitored in both cases, because the diurnal and seasonal cycles in these variables are often so large, they virtually swamp any longer-term, more slowly evolving timescale changes as they are taking place. Thus, shortterm climate change can often be identified using relatively coarse sampling resolution (and accompanying precision and accuracy); if longer-term change is to be detected using relatively short time series, these measures require considerably higher resolution (and precision and accuracy). How much higher changes with the variables measured and the rate at which the dec-cen change occurs. However, in any case, care must be taken to provide measurements of sufficient resolution and precision to allow extraction of the dec-cen signal at the earliest possible moment to make the most efficient use of the data. This consideration cannot be overlooked when designing joint monitoring sites geared toward weather and interannual and dec-cen studies.
In an analogous manner, the reliance of dec-cen studies on modeling demands considerable computing resources, since the models used in these efforts are often subject to long-term numerical drift in analyzing long timescales. This inadequacy simply reflects an inadequate treatment of the higher-order physics that often serve as the feedback mechanisms required to eliminate such drift. However, such higher-order physics typically involve more detailed regional or local-scale boundary interactions, which again require higher resolution, either in the vertical or horizontal dimension, or both. Over longer timescales the slower components of the system have an opportunity to become more intimately involved in climate evolution, so better treatment of additional components is also required. Further, the simulations themselves must involve much longer simulation times to resolve long timescales adequately. Obviously, the computer resources demanded by such models are extensive. Therefore, there must be a concerted effort to make the fastest computers readily available, so as to facilitate widespread access by a very broad and diverse modeling community. Sufficient resources are also needed so that the simulations required can be made as quickly and as often as needed.
Dec-cen climate studies are in their infancy, but advances and understanding are coming quickly. Because of the potential that climate change has to influence society dramatically over the timescale of a human life, we must make serious efforts to foster this research and build understanding to provide a sound scientific basis for national policy. Only then can policy makers take the necessary steps to ensure our long-term well-being—regardless of whether future climate changes are driven by natural or anthropogenic means.